This invention relates to a preformed yarn useful for the formation of thermoplastic, shaped, composite articles and to a method of producing same.
Well known composite materials include prepregs formed by coating a tow or a woven cloth of reinforcing fibers with a solution or a low-viscosity melt of a thermosetting resin. The prepreg, however, is extremely high in adhesiveness and poor in flexibility so that it has problems in handling and post processing. Also known is a tape of a reinforcing fiber-containing thermoplastic resin prepared by extruding a reinforcing fiber tape in which a thermoplastic resin having a high melting point is impregnated. This tape is, however, in the state of an extremely rigid board so that it causes difficulties in the formation of woven clothes and moreover it cannot be subjected to drape forming using molds with complex shapes.
In order to overcome these drawbacks, there has been proposed a flexible composite material (U.S. Pat. No. 4,614,678 issued to Ganga) which is formed by covering coarse fiber filaments, impregnated by a thermoplastic resin powder, with a flexible covering material. While this composite material is suitable for draping through molds with complex shapes in terms of flexibility as compared to known reinforcing fiber-containing thermoplastic resin tapes, it still suffers from the following drawbacks. Firstly, the composite material presents a difficulty in providing a uniform blend ratio of the reinforcing filaments and the thermoplastic resin powder, since the impregnation of the filaments with the powder is effected by contact of the filaments with a fluidized layer of the powder. As a result, the material properties of the resulting molded products obtained with such a composite material are not uniform. Secondly, in the production of the composite material, it is necessary to use thermoplastic resin powder whose average particle sizes distribute within a narrow range in order to maintain a stable fluidized state of the resin powder. This will increase the cost of production. Thirdly, since thermoplastic resin powder having a particle size greater than the diameter of the reinforcing filaments is used in relatively a large amount, the composite material becomes volumesome. This will cause a difficulty in handling during the drape forming of the material.